This invention relates to automated metrological machines used for optical inspection of an object. Metrological machines are typically employed for the automated optical inspection of manufactured objects, and are particularly useful in determining the precise dimensional measurements of such objects. They normally include a support on which the work object rests, and means for precisely moving either the object or an imaging video camera that is used for recording and/or displaying a magnified image of the object that is being inspected. These features allow such machines to perform precision measurements in the horizontal (or X-Y) plane. Autofocus means can also be included for determining heights of the object along the “Z” axis normal to the X-Y plane, enabling a full, three-dimensional inspection of the object.
A good example of an advanced metrological machine system is provided by U.S. Pat. No. 6,518,996 for a “Compact Video Inspection Apparatus with Y,Z,X Compounded Measurement Axes” (2003). In this patent, the object to be inspected is positioned on a support surface beneath a series of imaging lenses that are secured to a system support that overlies the support surface and object. The lenses have an optical axis disposed vertically with a front lens and either a fixed or a zooming system in the back focusing on a camera sensor (imaging plane).
A good example of a lens system used in such an inspection apparatus is provided by U.S. Pat. No. 6,292,306, for a “Telecentric Zoom Lens System for Video Based Inspection System” (2001). Collimated space behind the front lens provides room for the insertion of a beamsplitter, such as the beamsplitter 16 shown in FIG. 1 of '306. The beamsplitter allows for additional optical systems to be added to the system such as the coaxial through-the-lens (“TTL”) surface illumination system described in '306. This system is used to inject surface inspection illumination from an illumination source transverse to the optical axis along the optical axis through the lens (i.e., “TTL”). (See, e.g., “illuminator light source S” in U.S. Pat. No. 6,292,306 and its accompanying drawing figures).
The complete illumination system for an advanced metrological machine may also include profile (back light) illumination, and a fiber bundle ring light or a LED variable ring light (dark field) illumination. (See, e.g., U.S. Pat. Nos. 6,488,398; 5,690,417; and 6,179,439). The fiber bundle ring light and the LED variable ring light are optimized for the standard focal length of a 1× front objective lens (e.g., 100 mm effective focal length (EFL)). However, the coaxial illumination system previously discussed can create a “hot spot” at low magnification and a loss of contrast at high magnification with the light rays reflecting off the surfaces of the lens into the camera.
In addition, the LEDs of the LED variable ring light are focused onto the stage with a, typically, 76 mm EFL Fresnel lens. (See, U.S. Pat. No. 5,690,417). When, as is current practice, 2× attachment lenses are used in addition to the existing standard lens, a 2× lens adapter tube is needed to match the focus of the Fresnel lens. The addition of this adapter tube changes the characteristics of the optical system. The focal length of the front lens is no longer 50 mm (1× plus 2× attachment) and the system NA (numerical aperture) is lowered from the standard 0.2. Also, the front lens now blocks the inner ring of the LED ring light. However, without the adapter tube, the ring light focuses far below the front focus of the front lens, thus providing little or no illumination. And, in addition to matching the illuminator for more light, a large working distance is always a benefit for objects with a variation in feature heights. This will prevent collision with the lens when focusing on a lower feature of the object. (The light sources for the previously described system are, in most cases, white).
Even more problems arise when the above-described system is used with a through the lens (TTL) laser range sensor that emits a laser beam (e.g., typical wavelength 655 nm) through the front lens onto the object and collects the reflected light through the same lens. The sensor for the laser is different from the camera sensor. The laser system works best with the 2× lens characteristics. The larger the NA of the system, the better the resolution of the laser system. With the mechanical restriction on the diameter of the front lens and the necessity of the 50 mm focal length, the optimal NA is 0.22. Because of the input of the laser (which enters the optical system via a beamsplitter inserted behind the front lens, as described with reference to U.S. Pat. No. 6,292,306), the lens must be infinity corrected for optimum performance of the laser system as well as the main imaging system to be achieved.
I have, therefore, sought and invented a system with a 50 mm effective focal length replacement lens system self-corrected to be used with multiple back end systems—such as zoom lenses; color corrected for full white light illumination and color cameras (but which can also be used with monochromatic systems and black and white cameras); and infinity corrected to allow for the insertion of the tilted beam splitter in the collimated space to provide surface illumination and laser input/collection.
This replacement 2× lens system will take the place of the standard 1× lens-2× attachment lens combination. The effective focal length is 50 mm to be a true 2× of the standard 1×, 100 mm EFL lens. The full 22 mm diameter of the front lens space is utilized for the NA of 0.22 for best laser range sensor results.
The materials for the doublets of this new system were chosen to have a small transition, in the same direction, in index between the lens elements of the doublets and the cement joining them. This eliminates the need for index matching coating on the cemented surfaces of said lens. This index matching coating is necessary when there is a large difference between the index of the glass and that of the cement, or if both the glass indices surrounding the cement are higher in index than the cement. Without the coating, there are back reflections of light off these lens surfaces and back to the camera, adding to the “hot spot” of the system. Using glass types F5 (index of 1.6) and FK51 (index of 1.48) with cement (index of 1.5) in-between produces minimum back reflections.
The non-cemented air-glass surfaces of all of the lenses of this system except for one have multi-layer anti-reflection coating, for minimum reflection of white light—a necessary step in order to reduce the hotspot created by the surface illuminator. The concave curve on remaining lens is coated with a “v-coat” to block the back reflection of the laser light that is focused onto the camera. The curve, in relation to the imaging system, actually focuses the back reflection of the laser light off this surface into the camera plane of the video system.
The working distance of the lens system is increased from 28 mm to 38.75 mm (where 28 mm is the working distance that the old 1×-2× combination system provided). And, the position of the lens in the mount is optimized with respect to the Fresnel lens of the standard ring light illuminator so the focal planes of the new system and that of the ring light correspond for maximum illumination. Consequently, the Z-height of this system's focal plane is the same as that of the standard. In addition, the tapered end of the lens system keeps it from blocking any of the light from the LEDs.
In summary, this system cures many deficits found in the current 1×-2× combination, including specifically those growing out of its use with variable illumination sources commonly used with metrologic machines, such as TTL white light illuminators, TTL laser illuminators, and standard ring light illuminators.